It has been known to use wire screens made of mesh products for various purposes. Wire made from high carbon steel has been used as screens in mining, quarry, sand and gravel, steel milling, coal, coking and slag operations and other similar operations such as security barriers in areas requiring a high degree of security. It is important for these uses that the screen be made from high carbon steels because of their high degrees of hardness which gives highest resistance to wear and hence longer working life.
These high carbon steel wire screens have heretofore been manufactured by a process involving weaving pre-crimped wires into a mesh by the use of a loom. This prior art process usually involves laying a longitudinal wire across another to form the desired woven screen or mesh. If larger diameter wires are used, the required processes involve manually placing these wires one across the other. If comparatively small diameter wires are used, they can be woven automatically using relatively expensive machinery. These machines utilize substantially the same technique of weaving as does a textile machine which throws a loaded shuttle from side to side. This method, however, is relatively impractical with large diameter wires and thus limits the widespread use of this weaving method when strong mining and quarry screens are desired. Therefore, to use the prior art automatic weaving methods, one is limited to wires having substantially small diameters. In situations where a high carbon steel screen is desired having larger wire diameters, it is generally required to produce them manually. This can be a time-consuming and relatively expensive manufacturing process. There is, therefore, a major need for an automatic, economical and efficient process for the manufacture of high carbon wire mesh screens having larger wire diameters.
There have been several attempts at commercial manufacture of low carbon content wire mesh using various welding techniques. U.S. Pat. Nos. 3,405,743; 3,539,752; 3,734,383; 4,021,634 and 4,023,600 all relate to methods of producing wire mesh using low carbon wire material. While these patents disclose methods for forming wire mesh screens having increased wire diameters, none of them relates to the use of high carbon steel wire mesh. U.S. Pat. No. 3,539,752 is directed to the fabrication of steel mats. The metal mats and the area near the crossing points of the bars to be welded is subjected to a heating cycle comprised of a first heating period so as to gradually raise the metal to a predetermined temperature above ambient or room temperature, followed by a second cooling period to again gradually reduce the temperature to room temperature according to a continuous time function. The welding of the bars takes place during the brief interval or transition from the heating to the cooling period. The patent states that the heat for preheating the metal can be supplied by any suitable source such as hot air. This patent is not concerned with the use of high carbon steel nor are the problems associated with welding this type material discussed or recognized. In addition, the preheating temperature used in the process of this prior art patent is relatively low.
U.S. Pat. No. 4,021,634 is directed to a mesh welding machine for welding two cross groups of parallel wires. There is not any mention in the patent of welding high carbon wire screens nor of the problems associated therewith.
U.S. Pat. No. 3,405,743 is directed to an apparatus for fabricating a reinforcing mat in which large diameter rods are used.
U.S. Pat. No. 3,734,383 relates to friction welding of longitudinal wires of a mesh. There is not any disclosure in this patent of the use of high carbon steel.
A further welding machine is shown in U.S. Pat. No. 3,731,042 where each welding head in the machine consists of an electrode above the product plane and an electrode below the plane. These are interconnected by a piece which is rigid with the upper electrode and to which the lower electrode is movably mounted. Means are provided to vertically reciprocate below the lower electrode in order that each longitudinal wire is moved upwards against the transverse wire during welding.
U.S. Pat. No. 4,023,600 relates to a method of producing wire mesh using low carbon wire rod material.
Methods for producing steels of high strength and directed to welded wire netting are discussed in Canadian Patent Nos. 709,370; 830,261; 881,892 and 910,163.
U.S. Pat. No. 709,370 discloses a method of producing steels of high strength in which the steel is heated above its transformation temperature then quenched with the corresponding transformation to a metal consisting essentially of martensite. The metal is then tempered up to about 400.degree. F. and it is subjected to a stress having a value between the yield strength of the unstressed metal and the value required to effect a permanent strain of about 4% thereon. There is nothing in the patent regarding the process of welding high carbon wire steels.
U.S. Pat. No. 830,261 is directed to a machine for manufacturing welded wire netting and is particularly concerned with the rollers on the machine for making the wire mesh. Again, there is no disclosure of welding high carbon steel. Canadian Patent No. 881,892 again deals with a machine for making welded wire mesh and is directed to welding heads which are adjustable in such machines. U.S. Pat. No. 910,163 deals with the making of steel springs and wherein the welding wire is not mentioned.
A major reason why none of the above-discussed prior art deals with welding of high carbon steel is that in prior art attempts the wire has become weak and brittle in the vicinity of the weld. Thus, the processes involved with welding of wire have been limited to wires containing low carbon steel.
Welding operations usually necessitate the local application of heat, the amount of heat applied and the temperature locally attained depending upon the type of weld to be made. Thus, the temperature at the heated area may be only sufficient to render the meeting surfaces of the members semi-plastic as in forge welding or sufficient to melt the meeting surfaces thoroughly as in full fusion welding. Although in welding operations the heat is usually applied only to or adjacent to the surfaces to be united, metal adjacent to and remote from the heated surfaces is also heated by conduction from the directly heated surfaces. Such heating being uneven tends to warp and distort the welded article or, in some cases, cause brittleness.
The problem of shrinking and warping is particularly important in the fabrication of welded structures from large sheets and plates such as those which are now used in the construction of railway box cars, gondola cars and other rolling stock. As the metal sheets and plates used in this particular field are relatively thin, they are peculiarly susceptible to warping during welding as the small mass of metal is not strong enough to be resistant to stresses developed by the expansion and construction incidental to the localized application of heat to the metal plate or metal object.
As the grain size is influenced by the time interval during which the temperature is maintained in or above the critical range, it is desirable to cool the metal rapidly to below the critical range as soon as possible after it has been deposited. In cross wire welding similar to projection welding, a large current is passed through the two surfaces which are to be welded. While the passage of this current having a pulse time of part of a second, particularly at the weld interface, causes sufficient heat for the metal to fuse, severe temperature gradients occur. High carbon steel is generally defined as steel or steel alloys having 0.5% to 1.5% carbon. Steel below this amount usually 0.10% to 0.3% is referred to as low or mild carbon steel. Steel having carbon contents between these two forms is usually referred to with reference to the percent carbon and not as "high" or "low" carbon steel. With the exception of very low carbon steels, the effects of these gradients are deleterious. When it is attempted to weld high carbon steels (0.5 to 1.5%) in prior art processes, the adverse effects get worse as the carbon content in the steel is increased. The formation and dispersion in these prior art processes of various forms of carbon and iron known as pearlite, sorbitic pearlite, bainite, troostite, sorbite, cementite, austenite and martensite are considerable. This is undesirable because while it is desirable to form martensite which is the hardest of these listed constituents, the formation of those other than martensite can drastically reduce the hardness of the steel.
Ideally, the grade of steel used for wire screens is that which has the highest hardness and consequently the lowest wear rate which is deemed to have the longest screen life. This occurs when the carbon content of the steel is at least 0.7%. This hardness is related to iron-carbon constituent martensite which is the hardest of those listed above. The specification of the steel is covered by AISI No. 1070, 1074 and 1078 and the martensite becomes harder still when quenched from an elevated temperature. This quenching should be by oil rather than water as the rate of cooling with water is too severe and will induce cracks. There exists today a need for an automatic, effective and an economical welding process for manufacturing wire mesh screens having wider ranges of diameters and composed of high carbon steel.